1. Field of the Invention
The invention relates to a method for controlling a hydrodynamic retarder in a motor vehicle that can be mechanically disengaged via a disconnect clutch.
2. Description of the Related Art
In order to reduce the no-load losses of a hydrodynamic retarder to zero, it has already been suggested to disengage the hydrodynamic retarder during a non-braking mode by means of a disconnect clutch from the drive train which it is intended to brake in the braking-mode. During the transition from braking mode to non-braking mode, the working chamber is at the same time drained of working medium which, in the braking mode transfers a hydrodynamic circulation in the working chamber for transmission of torque from the rotor to the stator; or, in the case of a counter-rotating retarder, from the rotor to a counter-rotating rotor rotating in opposite direction, and therefore brakes the rotor and with it the driving drive train.
Turning on and turning off a hydrodynamic retarder is generally controlled by an electronic control device, whereby the electronic control device receives requests in regard to turning on or turning off; or in regard to adjustment of a certain braking torque or respectively a certain braking stage either from the vehicle's operator who operates a relevant input device, for example a control lever, or in certain arrangements also from a driver assist system, for example a speed control system or a distance maintaining system of the vehicle. When for example the electronic control device detects a request to turn off the retarder it will effect draining of the working chamber of working medium and disengaging of the disconnect clutch.
With regard to traffic safety and comfort it is important that demands in regard to turning on, turning off or adjusting of a certain braking torque of the hydrodynamic retarder are implemented as rapidly as possible. The goal is for example, implementation of a specific request in less than one second or even less than 0.5 seconds. It is therefore understandable that when, for example a request for turning off the retarder has been detected, draining of the working chamber and disengaging of the disconnect clutch is commonly implemented immediately and expediently.
In practical application it has been shown that the desired reaction times can generally be adhered to. However, especially with frequent turning on and turning off of hydrodynamic retarders that are mechanically disengaged from the drive train during the transition from braking mode to non-braking mode and are again connected during transition from non-braking mode to braking mode through frictional slip shunting in the disconnect clutch, the wear and tear in the clutch is relatively high and the maintenance cost accordingly great.
In the post-published German patent application DE 10 2011 120 626 it is therefore suggested to implement delayed disengaging of the disconnect clutch during the transition from braking mode to non-braking mode, whereby the time span of the delay occurs subject to various state variables of the hydrodynamic retarder. The number of necessary synchonizations of the disconnect clutch can thereby be reduced and the service life of the disconnect clutch be increased.
Even though the wear on the disconnect clutch can be considerably reduced through the aforementioned measures there are still situations where the disconnect clutch is subject to especially high wear, reducing its service life.
It is therefore the objective of the current invention to provide a method to control a hydrodynamic retarder that can be mechanically disengaged via a disconnect clutch, wherein the wear and tear of the disconnect clutch is reduced.